Motor vehicles are known that operate on compressed natural gas. CNG is routed through a restriction and pressure regulator to supply CNG at a constant pressure to the engine fuel injectors.
A problem with motor vehicles operating on CNG is that as the tank nears empty, CNG supply to the engine fuel injector is no longer at constant pressure. Consequently, accurate control of the amount of CNG delivered to the engine becomes difficult to control as the fuel tank empties resulting in poor drivability and difficult emission controls.
One approach to addressing this problem has been to require tank refill when CNG pressure (e.g. tank pressure or fuel injection pressure) falls below a predetermined value. This type of approach results in frequent tank refills and limitation of the vehicle range of operation.
In another approach, for vehicles that are capable of operating on either CNG or gasoline, CNG delivery is shut off and only gasoline supplied to the engine when the CNG tank pressure falls below a desired value. Here again, CNG which could otherwise operate the vehicle remains unused in the tank until the next refill.
The inventors herein have discovered the problem with the above approaches and solved the problem. In particular, flow rate through the restriction is at a choked sonic flow where flow velocity in the choked region remains constant at sonic velocity. The inventors further recognized that sonic choke flow rate (in mass terms) continues to decrease as tank pressure decreases. The solution recognized by the inventors is to compute the sonic choke mass flow rate and control the average gaseous injection rate to be always below this value. Accurate control of gaseous fuel injection into the engine is then achievable because when the fuel injection rate is less than the sonically limited mass flow rate, pressure regulation occurs. In a further example, the inventors substantially maximized mass flow rate to the fuel injector to substantially minimize time for fuel tank emptying while maintaining on average mass flow rate from the fuel injector to be less than mass flow rate of gaseous fuel delivered to the fuel injector through the restriction, which in one embodiment may be located in the regulator, but in other embodiments can also be located in other locations within the fuel system. In another example, the inventors added the injection of a liquid fuel into the engine through a liquid fuel injector; and controlled the gaseous fuel injector and the liquid fuel injector to substantially achieve a desired engine operation—such as desired air/fuel control or desired torque control.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the Detailed Description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.